1. Field of the Invention
This invention relates to an electrochromic device.
2. Related Background Art
A phenomenon of reversible coloration through the reversible electrolytic oxidation or reduction reaction which occurs by application of a voltage is called electrochromism. By use of an electrochromic (hereinafter abbreviated as EC) substance exhibiting such phenomenon, attempts have been made to prepare an EC device (hereinafter abbreviated as ECD) which is colored or decolored by voltage operation and utilize the ECD for dose controlling device (e.g. fender mirror) or for numeral displaying device utilizing 7 segments since 20 years or before. For example, an ECD comprising a lower electrode layer, either one of a tungsten trioxide thin film as the EC substance or an insulating film such as silicon dioxide, and then the other, and finally an upper electrode layer successively laminated on a glass substrate (U.S. Pat. No. 3,521,941) has been known as the whole solid type ECD.
Since each layer of ECD is extremely thin film formation is performed according to vacuum thin film forming technique such as the reactive or non-reactive vacuum vapor deposition, the reactive or non-reactive ion plating, the reactive or non-reactive sputtering, CVD.
When a voltage as obtainable from a cell is applied on such ECD, the tungsten trioxide (WO.sub.3) thin film is colored in blue. Then, when a reverse voltage to the same extent is applied on the ECD, the blue color of the WO.sub.3 film is extinguished to become colorless. The mechanism of such coloration-decoloration has not yet been clarified in detail, but it has been understood that a small amount of water contained in the WO.sub.3 thin film and/or the insulating film governs the coloration and decoloration of WO.sub.3. The reaction scheme of coloration is estimated as follows.
Cathode side: EQU H.sub.2 O .fwdarw.H.sup.+ +OH.sup.- EQU WO.sub.3 +nH.sup.+ +ne.sup.- .fwdarw.H.sub.n WO.sub.3
(colorless transparent):(blue)
Anode side: EQU OH.sup.- .fwdarw.1/2H.sub.2 O+1/40.sup.2 .uparw.+1/2e.sup.-.
Afterwards, the objective type ECD added with a layer for oxidative reaction compensating for the reduction reaction of the WO.sub.3 layer was invented (U.S. Pat. No. 4,350,414). FIG. 2 shows a sectional structure of such ECD.
At least one of a pair of electrode layers sandwiching the EC layer directly or indirectly therebetween must be transparent for permitting coloration and decoloration of the EC layer to appear to the outside. Particularly, in the case of the transmission type ECD, both must be transparent. As transparent electrode materials, there have been known up to date SnO.sub.2, In.sub.2 O.sub.3, ITO (mixture of SnO.sub.2 and In.sub.2 O.sub.3), ZnO, etc., but since these materials are relatively poor in transparency, they must be made thin. For this reason and other reasons, ECD is commonly formed on a substrate such as glass plate or plastic plate.
In FIG. 2, (A) shows a first electrode layer, (B) a reversible electrolytic oxidative layer or an oxidative colorable Ec layer (e.g., iridium oxideor hydroxide), (C) an ion electroconductive layer, (D) a reductive colorable EC layer (e.g. WO.sub.3) and (E) a second electrode layer, respectively, and ECD is constituted basically of the laminated structure (A)-(E). However, as described above, such ECD is formed on a substrate (S). In FIG. 2, (R) is an encapsulating material such as epoxy resin, and (G) an encapsulating substrate for protection.
For supplying external power sources to the electrodes (A) and (E) of such ECD, take-out electrodes are respectively required, to which external wirings (L.sub.A), (L.sub.E) are respectively connected.
Whereas, as is necessarily required in the case of the transmission type ECD, the upper second layer electrode layer must be made a transparent electrode. Further, for making the response of coloration and decoloration rapid and preventing generation of coloration irregularity, the transparent electrode must be made to have a resistance as low as possible. Particularyly, in an ECD with large display area, a second electrode layer with low resistance is preferred for the above characteristics.
However, although SnO.sub.2, In.sub.2 O.sub.3, ITO, ZnO, etc. have been known as transparent electrode materials, all of these materials will have high resistance when formed into films at relatively lower temperature, and low resistance when formed into films at relatively higher temperature.
In the case of the lower electrode layer, since the electrode layer is formed directly on the substrate, by use of the most general glass substrate as the substrate, which can stand high temperature, film formation can be effected at high temperature such as 300.degree. to 400.degree. C., whereby a transparent electrode layer with low resistance (which exhibits sheet resistance of 10 ohm or less) can be obtained. Here, sheet resistance refers to a resistance value between two electrodes when electrodes are respectively provided at one side of a layer shaped in a square with one side of 1 cm and at the other side opposed to said one side, and the thickness of the square layer is about 2000 .ANG..
In contrast, in the case of the upper electrode layer, various layers have been already formed on the substrate. Accordingly, when the temperature is made higher for the purpose of making lower the resistance of the upper electrode layer, the EC layer is exposed to high temperature, whereby there is involved the problem that the EC layer is remarkably deteriorated and, in an extreme case, ECD can be no longer actuated.
On the contrary, if the upper transparent electrode layer is formed at a low temperature which will not deteriorate the EC layer, a transparent electrode layer with higher resistance (which exhibits sheet resistance of 50 ohm or higher) is obtained, thus, giving rise to a problem that response of ECD is slow to generate coloration irregularity. This problem is particularly marked when the display area is large.